better directory handling

Rename Locations functions for better consitency, and make their values
more consistent too.

Used </> rather than manually building paths. There are still more places
that manually do so, but are tricky, due to the behavior of </> when
the second FilePath is absolute. So I only changed places where
it obviously was relative.

Command/Unlock.hs

@@ -43,7 +43,7 @@ perform dest key = do
 		error "content not present"
 
 	g <- Annex.gitRepo
-	let src = annexLocation g key
+	let src = gitAnnexLocation g key
 	liftIO $ removeFile dest
 	showNote "copying..."
 	ok <- liftIO $ copyFile src dest